19-1454; Rev 0; 4/99 L MANUA ION KIT HEET T A U L EVA TA S WS DA FOLLO Low-Noise, 14V Input, 1A, PWM Step-Down Converters The MAX1684/MAX1685 are high-efficiency, internalswitch, PWM step-down switching regulators intended to power cellular phones, communicating PDAs, and handy-terminals. These devices deliver a guaranteed 1A output current from two Li-Ion batteries. Their wide input voltage range of 2.7V to 14V gives design flexibility and allows batteries to charge from a wall cube, since the ICs operate at the higher voltages that occur when the battery is removed. The output voltage is preset to 3.3V or can be externally adjusted from 1.25V to VIN. The low-on-resistance power switch and built-in synchronous rectifier provide high efficiencies of up to 96%. There are four modes of operation: fixed-frequency mode, normal mode, low-power mode, and shutdown mode. The fixed-frequency PWM mode of operation offers excellent noise characteristics. The normal mode maintains high efficiency at all loads. The low-power mode is used to conserve power in standby or when full load is not required. The shutdown mode is used to power down the device for minimal current draw. The MAX1684 runs at 300kHz for applications that require highest efficiency. The MAX1685 runs at 600kHz to allow use of smaller external components. These devices can also be synchronized to an external clock. Other features include a 100% duty cycle for low-dropout applications, an auxiliary 3V/5mA output, and a 1% accurate reference. Both devices are available in a space-saving 16-QSOP package. An evaluation kit is also available to help speed designs. For a similar device in a 10-pin µMAX package with lower input voltage requirements (5.5V max), see the MAX1692 data sheet. Features ♦ Up to 96% Efficiency ♦ 1A Guaranteed Output Current ♦ 100% Duty Cycle in Dropout ♦ 2.7V to 14V Input Range (15V absolute max) ♦ ±1% Accurate Reference Output ♦ 0.24Ω P-Channel On-Resistance ♦ Synchronizable Switching Frequency ♦ Fixed-Frequency PWM Operation 300kHz (MAX1684) 600kHz (MAX1685) ♦ 150µA Normal Mode Quiescent Current ♦ 25µA Low-Power Mode Quiescent Current ♦ 2µA Shutdown Current ♦ Dual Mode™ Fixed 3.3V (±1%) Output or Adjustable Output (1.25V to VIN) ♦ Small 16-QSOP Package ♦ Auxiliary Output (CVL): 3V/5mA Ordering Information TEMP. RANGE PIN-PACKAGE MAX1684EEE PART -40°C to +85°C 16 QSOP MAX1685EEE -40°C to +85°C 16 QSOP Typical Operating Circuit Applications Cellular Phones OUTPUT 3.3V AT 1A INPUT 2.7V TO 14V 2-Way Radios and Walkie-Talkies Computer Peripherals Personal Communicators PDAs and Handy-Terminals + IN LX AIN SHDN MAX1684 MAX1685 + GND CVH CVL BOOT STBY SYNC/PWM Pin Configuration appears at end of data sheet. FB CC REF Dual Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1684/MAX1685 General Description MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters ABSOLUTE MAXIMUM RATINGS AIN to AGND ............................................................-0.3 to +15V IN to PGND ................................................-0.3V to (VAIN + 0.3V) LX to PGND .................................................-0.5V to (VIN + 0.3V) PGND to AGND ..................................................................±0.3V SHDN to AGND .........................................-0.3V to (VAIN + 0.3V) ILIM/SS, FB, CC, BOOT, REF to AGND .....-0.3V to (VCVL+ 0.3V) CVH to IN..................................................................-6V to +0.3V CVL, STBY, SYNC/PWM to AGND............................-0.3V to +6V Reference Current ..............................................................±1mA CVL Current ..........................................................10mA to -1mA LX Peak Current (Internally Limited) .....................................2.3A Continuous Power Dissipation (TA = +70°C) 16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10sec ) ............................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = V SHDN = +6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS Input Voltage Range Feedback Voltage Output Voltage (3.3V mode) VFB VOUT VFB = VOUT, ILOAD = 0 to 1A FB = AGND, ILOAD = = 0 to 1A VFB = VOUT, ILOAD = 0 to 1A Output Current Capability VIN = 5V to 14V Output Adjust Range BOOT = AGND (Note 1) IFB On-Resistance, N-Channel Current Limit in Low-Power Mode Current Limit, N-Channel Zero Crossing Threshold 2 V 1.251 1.264 3.3 3.333 3.365 0.01 V % A VREF VIN V -50 50 nA 0.24 0.5 VIN = 2.7V 0.34 0.8 Ω 3 8 Ω 1.2 1.75 2.3 A SYNC/PWM = low 285 380 475 mA STBY = low 285 380 475 mA A SYNC/PWM = high 0.15 0.4 0.9 MAX1684 20 50 80 MAX1685 40 80 120 MAX1684 13 33 MAX1685 25 65 Normal mode, SYNC/PWM = low, VBOOT = 3.3V (Note 2) 0.9 2 Low-power mode, STBY = low, VBOOT = 3.3V (Note 2) 0.14 0.27 SYNC/PWM = low PWM mode, SYNC/PWM = high, VBOOT = 3.3V (Note 2) Quiescent Power Consumption V 1.238 Low-side switch, VIN = 2.7V, ILX = 200mA ILIMLP UNITS 14 VIN = 6V ILIM Pulse-Skipping Current Threshold MAX 1 VFB = 1.4V High-side switch, ILX = 1A On-Resistance, P-Channel Current Limit in PWM Mode TYP 2.7 Output Load Regulation FB Input Current MIN mA mW _______________________________________________________________________________________ Low-Noise, 14V Input, 1A, PWM Step-Down Converters (VIN = V SHDN = +6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Quiescent Supply Current in Dropout Oscillator Frequency SHDN = low ILX fOSC SYNC Capture Range TYP MAX UNITS 230 430 µA 6 µA 20 µA 2 VIN = 14V, VLX = 0 or 14V, SHDN = low MAX1684 260 300 340 MAX1685 520 600 680 MAX1684 180 350 MAX1685 360 700 Maximum Duty Cycle 100 Constant-Frequency Minimum Duty Cycle Reference Output Voltage MIN STBY = low, VIN = 2.7V Shutdown Supply Current LX Leakage Current CONDITIONS (Note 3) VREF 10 MAX1685 20 1.238 1.264 V 4 15 mV 0.2 5 mV 3.0 3.15 V 120 mV 2.5 2.6 V -5.0 -4.6 -4.1 V 2.4 2.5 2.6 V 4.65 µA -1µA < IREF < 50µA Reference Supply Regulation 2.7V < VBOOT < 5.5V CVL Regulator Output Voltage VIN = 3V to 14V, BOOT = AGND, ICVL = 0 to 5mA CVL Dropout Voltage BOOT = AGND, ICVL = 5mA CVL Undervoltage Lockout Threshold BOOT = AGND, CVL falling edge, typical hysteresis is 40mV 2.4 CVH with Respect to VIN ICVH = -1mA BOOT Switchover Threshold BOOT falling edge, typical hysteresis is 0.1V Thermal Shutdown Threshold Typical hysteresis is 10°C (Note 4) ILIM/SS Source Current VILIM/SS = 1.4V VIH Logic Input Low Voltage VIL SHDN, STBY, SYNC/PWM Logic Input Current SHDN, STBY, SYNC/PWM SYNC/PWM Pulse Width High or low period % 1.251 Reference Load Regulation Logic Input High Voltage kHz % MAX1684 IREF = 0 kHz 2.7 160 3.3 4 °C 2 -1 500 V 0.7 V 1 µA ns _______________________________________________________________________________________ 3 MAX1684/MAX1685 ELECTRICAL CHARACTERISTICS (continued) MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters ELECTRICAL CHARACTERISTICS (VIN = V SHDN = +6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = -40°C to +85°C, unless otherwise noted.) (Note 5) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 2.7 14 V VFB = VOUT, ILOAD = 0 to 1A 1.236 1.266 V FB = AGND, ILOAD = = 0 to 1A 3.295 3.375 Input Voltage Range Output Feedback Voltage Output Voltage (3.3V mode) VFB VOUT Output Current Capability VIN = 6V to 14V Output Adjust Range BOOT = AGND (Note 1) FB Input Current IFB Current Limit in PWM Mode ILIM Current Limit in Low-Power Mode ILIMLP STBY = low VREF V -50 50 nA 1.2 2.3 A 285 475 mA 2 mW Low-power mode, STBY = low, VBOOT = 3.3V (Note 2) 0.27 SHDN = low Shutdown Supply Current fOSC V A VIN Normal mode, SYNC/PWM = low, VBOOT = 3.3V (Note 2) Quiescent Power Consumption Oscillator Frequency VFB = 1.4V 1 6 µA MAX1684 240 350 MAX1685 480 700 1.236 1.266 V kHz Reference Output Voltage IREF = 0 CVL Regulator Output Voltage VIN = 3V to 14V, BOOT = AGND, ICVL = 0 to 5mA 2.7 3.15 V CVL Undervoltage Lockout Threshold BOOT = AGND, CVL falling edge, typical hysteresis is 40mV 2.4 2.6 V CVH with Respect to VIN ICVH = -1mA -5.0 -4.1 V BOOT Switchover Threshold BOOT falling edge, typical hysteresis is 0.1V 2.4 2.6 V ILIM/SS Source Current VILIM/SS = 1.4V 3.1 4.7 µA Logic Input High Voltage VIH Logic Input Low Voltage VIL SHDN, STBY, SYNC/PWM 2 0.7 V Note 1: The output adjust range with BOOT connected to VOUT is VREF to 5.5V. Connect BOOT to AGND for VOUT > 5.5V. Note 2: The quiescent power-consumption specifications include chip supply and gate-drive loss only. Divide these values by VIN (= 6V) to obtain quiescent currents. In normal and low-power modes, chip supply current dominates and quiescent power is proportional to VBOOT (BOOT connected to OUT). In PWM mode, gate-drive loss dominates and quiescent power is proportional to VIN · (VIN - VCVH). In addition, IR losses in power switches and external components typically increase PWM quiescent power consumption by 5mW to 10mW. Note that if the device is not bootstrapped, additional power will be dissipated in the CVL linear regulator. Note 3: When the duty factor (VOUT / VIN) is less than this value, the switching frequency decreases in PWM mode to maintain regulation. Note 4: Thermal shutdown is disabled in low-power mode (STBY = low) to reduce power consumption. Note 5: Specifications to -40°C are guaranteed by design, not production tested. 4 _______________________________________________________________________________________ Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684 EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V, VOUT = 1.8V, 2.5V) C B A: VOUT = 2.5V LP MODE B: VOUT = 1.8V LP MODE C: VOUT = 2.5V NORM MODE D: VOUT = 1.8V NORM MODE E: VOUT = 2.5V PWM MODE F: VOUT = 1.8V PWM MODE F 30 20 10 0 0.1 1 10 100 1,000 50 40 30 10 MAX1684/85-03 A: VIN = 4V B: VIN = 5V C: VIN = 9V D: VIN = 12V 20 10 0 0 0.1 1 10 100 1,000 1 10,000 10 100 1,000 10,000 MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V, PWM MODE) MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, PWM MODE) E F 60 50 A: VIN = 6V LP MODE B: VIN = 9V LP MODE C: VIN = 12V LP MODE D: VIN = 6V NORMAL MODE E: VIN = 9V NORMAL MODE F: VIN = 12V NORMAL MODE 30 20 0.1 1 10 100 1,000 80 70 60 50 B 40 C A: VIN = 6V B: VIN = 9V C: VIN =12V 20 10 10 50 40 100 1,000 A: VIN = 4V LP MODE B: VIN = 12V LP MODE C: VIN = 4V NORMAL MODE D: VIN = 12V NORMAL MODE 20 10 0 1 10 100 LOAD CURRENT (mA) 1,000 10,000 VIN =9V 20 VIN = 12V 1 10,000 100 MAX1684/85-08 VIN = 9V 60 50 40 VIN =12V 80 C 60 40 A: VIN = 6V LOW-POWER MODE B: VIN = 9V LOW-POWER MODE C: VIN = 12V LOW-POWER MODE D: VIN = 6V NORMAL MODE E: VIN = 9V NORMAL MODE F: VIN =12V NORMAL MODE 0 0 10 100 1,000 LOAD CURRENT (mA) 10,000 D E 50 20 10 F 70 10 1 10,000 B A 90 30 20 1,000 MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V) VIN = 6V 70 100 MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V PWM MODE) 90 80 10 LOAD CURRENT (mA) 30 30 40 LOAD CURRENT (mA) 100 EFFICIENCY (%) B 60 VIN = 5V 50 0 1 D 60 10 0 MAX1684/85-07 80 70 30 MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) C VIN = 4V 80 30 10,000 A 90 A LOAD CURRENT (mA) 100 100 MAX1684/85-06 90 EFFICIENCY (%) E 100 EFFICIENCY (%) D 40 EFFICIENCY (%) 40 MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V) 70 0.1 50 LOAD CURRENT (mA) C 70 B 30 A: VIN = 4V LOW-POWER MODE B: VIN = 12V LOW-POWER MODE C: VIN = 4V NORMAL MODE D: VIN = 12V NORMAL MODE 20 D 60 LOAD CURRENT (mA) A 90 MAX1684/85-02 60 C A 70 LOAD CURRENT (mA) B 80 70 10,000 100 90 80 D MAX1684/85-05 40 E EFFICIENCY (%) 50 B 90 MAX1684/85-09 D 60 80 100 EFFICIENCY (%) A C A 90 MAX1684/85-04 EFFICIENCY (%) 80 EFFICIENCY (%) 90 EFFICIENCY (%) 100 MAX1684/85-01 100 70 MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, PWM MODE) MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) 0.1 1 10 100 1,000 10,000 LOAD CURRENT (mA) _______________________________________________________________________________________ 5 MAX1684/MAX1685 Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) SAFE OPERATING AREA 1.5 1.4 1,000 800 600 1.3 400 1.2 200 1.1 2 4 6 VOUT = 3.3V 200 VOUT = 5V 100 0 PWM OR NORMAL MODE 0 1.0 0 300 8 10 12 4 2 6 8 10 12 14 INDUCTOR RESISTANCE INCLUDED 16 0 INPUT VOLTAGE (V) 14 0 200 INPUT VOLTAGE (V) MAX1684 NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (VOUT = 3.3V, PWM MODE) NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (VOUT = 3.3V) 150 NORMAL MODE 50 800 1,000 LOW-POWER MODE MAX1684/85-14 15 4.5 14 4.0 3.5 3.0 MAX1684 13 INPUT VOLTAGE (V) 200 100 600 PWM FIXED-FREQUENCY OPERATION AREA 5.0 SUPPLY CURRENT (mA) SCHOTTKY LEAKAGE CURRENT INCLUDED MAX1684/85-13 300 250 400 LOAD CURRENT (mA) M AX1684/85-15 LOAD CURRENT (A) 1.6 DROPOUT VOLTAGE vs. LOAD CURRENT 400 DROPOUT VOLTAGE (mV) MAX1684/85-10 1.7 VOUT = 3.3V 1,200 LOAD CURRENT (mA) 1.8 MAX1684/85-11 1,400 MAX1684/85-12 MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE SUPPLY CURRENT (µA) MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters 12 MAX1685 11 10 9 8 7 2.5 6 0 4 5 6 7 8 9 10 11 12 13 14 15 2.0 2.7 4 INPUT VOLTAGE (V) 5 6 7 8 9 10 11 12 0 INPUT VOLTAGE (V) LOAD-TRANSIENT RESPONSE ILOAD 500mA/div 4 6 8 10 12 SWITCHING WAVEFORM MAX1684/85-18 MAX1684/85-17 VOUT 20mV/div VLX 5V/div VOUT 50mV/div ILX 100mA/div 2ms/div MAX1684, ILOAD = 0.1mA TO 1A, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V 6 14 OUTPUT VOLTAGE (V) SWITCHING WAVEFORM MAX1684/85-16 2 ILX 100mA/div 1µs/div 1µs/div MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V _______________________________________________________________________________________ Low-Noise, 14V Input, 1A, PWM Step-Down Converters START-UP CURRENT LINE-TRANSIENT RESPONSE MAX1684/85-20 MAX1684/85-19 VSHDN 5V/div VIN 5V/div 0V VOUT 100mV/div IIN 50mA/div 10ms/div 2ms/div MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, CILIM/SS = 0.1µF, SYNC/PWM = 3.3V MAX1684, ILOAD = 100mA, VIN = 5V TO 10V, SYNC/PWM = 3.3V Pin Description PIN NAME FUNCTION 1 CVH High-Side MOSFET Gate Bias. Bias voltage for P-channel switch. Bypass to IN with a 0.1µF capacitor. 2 AIN Analog Supply Voltage Input. Connect to IN with a 0.2 in. metal trace. Bypass to PGND with a 0.1µF capacitor. 3 IN 4 CVL 5 AGND 6 REF Reference Output. 1.25V reference output supplies 10µA for external loads. Bypass to AGND with 0.1µF capacitor. 7 FB Dual Mode Feedback Input. Connect FB to VOUT for 1.25V output. Connect to an external resistor divider to adjust the output voltage. Connect to AGND to set output voltage to 3.3V. 8 CC Integrator Capacitor Connection. Connect a 0.01µF capacitor to AGND. Supply Voltage Input Logic Supply Voltage Output and IC Logic Supply. Sources 5mA for external loads. Bypass to AGND with 1µF capacitor. Analog Ground SYNC/PWM Input: For synchronized-PWM operation, drive with TTL level, 50% square wave. Connect to CVL for PWM mode. Connect to AGND for normal mode. 9 SYNC/PWM 10 ILIM/SS 11 STBY Standby Control Input. Connect to CVL for normal operation. Connect to AGND for low-power mode (Table 1). This pin overrides SYNC/PWM setting. 12 BOOT Bootstrap Input. Connection for the bootstrap switch and internal feedback path. Connect BOOT to VOUT for VOUT < 5.5V. Connect BOOT to AGND for VOUT > 5.5V. Current-Limit Adjust/Soft-Start Input. See Current Limit and Soft-Start section. 13, 14 LX 15 SHDN Inductor Connection. Drain for internal P-channel MOSFETs. Connect inductor from LX to OUT. Active-Low Shutdown Input. Connect to ground for shutdown. SHDN can withstand the input voltage. 16 PGND Power Ground _______________________________________________________________________________________ 7 MAX1684/MAX1685 Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters 0.1µF INPUT 14V MAX 3 1 CVH IN LX L 10µH* 13, 14 OUTPUT 3.3V AT 1A COUT 100µF cations. A SYNC input allows synchronization to an external clock. The MAX1684/MAX1685 can operate in five modes. Setting the devices to operate in the appropriate mode for the intended application (Table 1) achieves highest efficiency. PWM Control _______________Detailed Description The MAX1684/MAX1685 use an oscillator-triggered minimum/maximum on-time current-mode control scheme (Figure 2). The minimum on-time is typically 220ns unless the regulator is in dropout. The maximum on-time is 2 / f OSC, allowing operation to 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load- and line-transient response. At each falling edge of the internal oscillator, the internal P-channel MOSFET (main switch) turns on. This allows current to ramp up through the inductor to the load and stores energy in a magnetic field. The switch remains on until either the current-limit comparator trips, the maximum on-time expires, or the PWM comparator signals that the output is in regulation. When the switch turns off during the second half of each cycle, the inductor’s magnetic field collapses, releasing the stored energy and forcing current through the output diode to the output filter capacitor and load. The output filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, smoothing the voltage across the load. The MAX1684/MAX1685 step-down, pulse-width-modulation (PWM) DC-DC converters provide an adjustable output from 1.25V to the input voltage. They accept inputs from 2.7V to 14V and deliver up to 1.6A. An internal MOSFET and synchronous rectifier reduce PC board area while maintaining high efficiency. Operation with up to 100% duty cycle minimizes dropout voltage. Fixed-frequency PWM operation reduces interference in sensitive communications and data-acquisition appli- During normal operation, the MAX1684/MAX1685 regulate the output voltage by switching at a constant frequency and modulating the power transferred to the load on each cycle using the PWM comparator. A multiinput comparator sums three weighted differential signals (the output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp) and changes states when a threshold is reached. It modulates output power by adjusting the 22µF 2 MBRS 130LT3 AIN 0.1µF MAX1685 PGND ON/OFF 15 1µF AGND SHDN 4 CVL 11 STBY 9 SYNC/PWM BOOT ILIM/SS 10 REF 6 0.1µF (OPTIONAL) FB CC 8 16 5 12 7 0.1µF 0.01µF *SUMIDA CD54-100; USE 22µH FOR MAX1684 Figure 1. Standard Application Circuit Table 1. Operating Modes STBY SHDN H H H Fixed-Frequency PWM 1.6A Clocked H H Fixed-Input Clock-Frequency PWM 1.6A Normal L H H PFM at light loads (<150mA). Fixedfrequency PWM at heavy loads (>150mA). 1.6A Low Power X L H Low-Power or Standby Mode. Shutdown X X L Circuit Disabled PWM Sync PWM 8 TYPICAL OUTPUT CAPABILITY SYNC/PWM MODE FUNCTION _______________________________________________________________________________________ 160mA 0 Low-Noise, 14V Input, 1A, PWM Step-Down Converters CVL IN MAX1684 MAX1685 4µA AIN SHDN CVH VH VL THERMAL SHUTDOWN MAX1684/MAX1685 ILIM/SS ILIM THRESHOLD ON CVL REF PFM CURRENT COMPARATOR REF ILIM COMPARATOR LX CONTROL & DRIVER LOGIC UNDERVOLTAGE COMPARATOR 2.5V SYNC/PWM STBY PWM COMPARATOR OSC SYNC SLOPE COMPENSATION & STANDBY PWM MODE CONTROL NORMAL MODE LOW-POWER MODE PGND ZERO-CROSSING COMPARATOR CVL 2.5V CC PFM COMPARATOR BOOT GM INTEGRATOR FB 0.125V AGND Figure 2. Functional Diagram _______________________________________________________________________________________ 9 MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters inductor peak current during the first half of each cycle, based on the output error voltage. The MAX1684/ MAX1685’s loop gain is relatively low to enable the use of a small, low-valued output filter capacitor. The 1.4% transient load regulation from 0 to 1A is compensated by an integrator circuit that lowers DC load regulation to 0.01% typical. Slope compensation accounts for the inductor-current waveform’s down slope during the second half of each cycle, and eliminates the inductorcurrent staircasing characteristic of current-mode controllers at high duty cycles. PFM Control In low-power mode, the MAX1684/MAX1685 devices switch only as needed to service the load. This reduces the switching frequency and associated losses in the P-channel switch, the synchronous rectifier, and the external inductor. During this PFM operation, a switching cycle initiates when the PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output-filter capacitor and load. The MAX1684/MAX1685 then wait until the PFM comparator senses a low output voltage again. In normal mode at light load (<150mA), the device also operates in PFM. The PFM current comparator controls both entry into PWM mode and the peak switch current during PFM operation. Consequently, some jitter is normal during transition from PFM to PWM with loads around 150mA, and it has no adverse impact on regulation. 100% Duty-Cycle Operation As the input voltage drops, the duty cycle increases until the P-channel MOSFET turns on continuously, achieving 100% duty cycle. Dropout voltage in 100% duty cycle is the output current multiplied by the onresistance of the internal switch and inductor, approximately 0.35V (IOUT = 1A). Very Low Duty-Cycle Operation Because of the P-channel minimum on-time and deadtime (duration when both switches are off), the MAX1684/MAX1685’s switching frequency must decrease in PWM or normal mode to maintain regulation at a very low duty cycle. The total P-channel on-time and dead-time is 290ns typical. As a result, the MAX1684/ MAX1685 will maintain fixed-frequency regulation at no load for VIN up to 10VOUT and 5VOUT, respectively (see PWM Fixed-Frequency Operation Area graph in the Typical Operating Characteristics). For higher VIN at no 10 load, the frequency decreases based on the following equation: f = VOUT / (VIN · 290ns) At medium- to full-load current (>100mA), V IN can increase slightly higher before the frequency decreases. Synchronous Rectification Although the primary rectifier is an external Schottky diode, a small internal N-channel synchronous rectifier allows PWM operation at light loads. During the second half of each cycle, when the inductor current ramps below the zero-crossing threshold or when the oscillator period ends, the synchronous rectifier turns off. This keeps excess current from flowing backward through the inductor. Choose an appropriate inductor to limit the PWM ripple current through the N-channel FET to 400mAp-p. Current Limit and Soft-Start The voltage at ILIM/SS sets the PWM current limit (ILIM = 1.75A) and the low-power current limit (ILIMLP = 380mA). The PWM current limit applies when the device is in PWM mode, in synchronized PWM mode, or delivering a heavy load in normal mode (Table 1). The ILIMLP limit applies when the device is in low-power mode. An internal 4µA current source pulls ILIM/SS up to CVL. To use the maximum current-limit thresholds, leave ILIM/SS unconnected or connect it to a soft-start capacitor. Connect an external resistor from ILIM/SS to AGND to adjust the current-limit thresholds. The PWM current-limit threshold is (I LIM · RILIM/SS · 4µA) / VREF and is adjustable from 0.5A to 1.75A. The low-power current-limit threshold is equal to (ILIMLP · RILIM/SS · 4µA) / VREF and is adjustable from 110mA to 380mA. For example, when RILIM/SS is 156kΩ, the PWM current limit threshold is 0.88A and the low-power current limit threshold is 0.19A. Connect a low-value capacitor from ILIM/SS to AGND to achieve soft-start, limiting inrush current. ILIM/SS internally shorts to AGND in shutdown to discharge the soft-start capacitor. Do not connect ILIM/SS to REF or CVL. Determine the soft-start duration by: tSOFT-START = CILIM/SS(1.25V / 4µA) where tSOFT-START is the time from SHDN going high to the regulator being able to supply full load current. For example, a 0.1µF capacitor yields 31ms of soft-start. ______________________________________________________________________________________ Low-Noise, 14V Input, 1A, PWM Step-Down Converters VOUT R1 MAX1684 MAX1685 C1 FB R2 ILIMLP = current limit in low-power mode. Internal Low-Voltage Regulators and Bootstrap (BOOT) The MAX1684/MAX1685 have two internal regulators (VH and VL) that generate low-voltage supplies for internal circuitry (Functional Diagram). The VH regulator generates -4.6V with respect to IN to supply the Pchannel switch and driver. Bypass CVH to IN with a 0.1µF capacitor. The VL regulator generates a 3V output at CVL to supply internal low-voltage blocks, as well as the N-channel switch and driver. Bypass CVL to AGND with a 1µF capacitor. To reduce the quiescent current in low-power and normal modes, connect BOOT to OUT. After start-up, when V BOOT exceeds 2.6V, the internal bootstrap switch connects CVL to BOOT. This bootstrap mechanism causes the internal circuitry to be supplied from the output and thereby reduces the input quiescent current by a factor of VOUT / VIN. Do not connect BOOT to OUT if the output voltage exceeds 5.5V. Instead, connect BOOT to AGND to keep CVL regulated at 3V. Apply an external supply voltage, such as +3.3V or +5V, to BOOT to achieve a bootstrap effect when the output voltage is too low to bootstrap the device (below 2.7V). CVL has a 5mA capability to supply external logic circuitry and is disabled in shutdown mode. ________________Applications Information Output Voltage Selection Connect FB to AGND to select the internal 3.3V output mode. Connect BOOT to OUT in this configuration. To select an output voltage between 1.25V and V IN, connect FB to a resistor voltage divider between the output and AGND (Figure 3). Select R2 in the 20kΩ to 100kΩ range. Calculate R1 as follows: R1 = R2 [( VOUT / VFB) - 1] where VFB = 1.25V. Figure 3. Setting Output Voltage Connect a small capacitor across R1 to compensate for stray capacitance at the FB pin. C1 = 5 (10−7) R2 For R2 = 100kΩ, use 4.7pF. Inductor Selection The MAX1684/MAX1685’s high switching frequency allows the use of small surface-mount inductors. Table 2 shows a selection of suitable inductors for different output voltage ranges. Calculate the minimum inductor by: L = 0.9(VOUT - 0.3V) / (IRIPPLE MAX · fOSC) where: IRIPPLE MAX = should be less than or equal to 400mA fOSC = 300kHz (MAX1684) or 600kHz (MAX1685) Capacitor Selection Select input and output filter capacitors to service inductor currents while minimizing voltage ripple. The input filter capacitor reduces peak currents and noise at the voltage source. The MAX1684/MAX1685’s loop gain is relatively low, to enable the use of small, low-value output filter capacitors. Higher capacitor values provide improved output ripple and transient response. Low-ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Avoid ordinary aluminum electrolytic capacitors, as they typically have high ESR. Low-ESR aluminum electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for space-constrained surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors ______________________________________________________________________________________ 11 MAX1684/MAX1685 The output current capability for each mode is determined by the following equations: IOUTMAX = ILIM - 0.5 · IRIPPLE (for PWM and normal modes) IOUTMAX = 0.5 · ILIMLP (for low-power mode) where: IRIPPLE = ripple current = (VIN - VOUT) · VOUT / (VIN · fOSC · L) ILIM = current limit in PWM mode MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters offer the lowest ESR overall. Sanyo OS-CON capacitors have the lowest ESR of the high-value electrolytic types. Use ceramic and OS-CON capacitors for very compact, high-reliability, or wide-temperature applications, where expense is justified. When using very lowESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response, and increase the output compensation capacitor if needed. Table 3 lists suppliers for the various components used with the MAX1684/MAX1685. Ensure that the minimum capacitance value and maximum ESR values are met: COUT > IOUT MAX / (VOUT · AC Load Reg · fOSC) where I OUT MAX = 1A, AC Load Reg ≅ 1.4%, and fOSC = 300kHz (MAX1684) or 600kHz (MAX1685). Output Diode Selection Use a 1A external Schottky diode (MBRS130LT3 or equivalent) for the output rectifier to pass inductor current during the start of the second half of each cycle. This diode operates before the internal N-channel MOSFET completely turns on and during high-current operation. Use a Schottky diode to avoid forward biasing the internal body diode of the N-channel MOSFET. RESR < 2 · AC Load Reg · VOUT/IOUT MAX Table 2. Inductor and Minimum Output Capacitor Selection MAX1684 (300kHz) VOUT (V) MAX1685 (600kHz) L (µH) MIN COUT (µF) L (µH) MIN COUT (µF) 1.25 to 2.7 22 220 10 100 2.7 to 4 22 100 10 47 4 to 6 47 68 22 33 6 to 14 68 47 33 22 Table 3. Component Suppliers SUPPLIER PHONE FAX 0.1µF CAPACITORS AVX Matsuo 803-946-0690 714-969-2591 803-626-3123 714-960-6492 Sanyo 619-661-6835 619-661-1055 Sprague 603-224-1961 603-224-1430 AIN VIN MAX1684 MAX1685 22µF IN 847-956-0702 Coilcraft 847-639-6400 847-639-1469 Murata-Erie 814-237-1431 814-238-0490 Sumida 847-956-0666 847-956-0702 TDK 847-390-4373 847-390-4428 SYNC/PWM 1µF REF DIODES 12 FB STBY R2 CVL 847-956-0666 0.1µF ILIM/SS 602-994-6430 BOOT PGND AGND CC 0.01µF VIN, MAX = 14V - |VOUT| 602-303-5454 100µF L R1 CVH INDUCTORS Motorola LX C1 0.1µF SHDN Sumida -VOUT -1.25V TO -5.5V MBRS 130LT3 0.01µF ( R2 -VOUT = -1.25V R1 + 1 Figure 4. Inverting Output ______________________________________________________________________________________ ) Low-Noise, 14V Input, 1A, PWM Step-Down Converters PC Board Layout High switching frequencies and large peak currents make PC board layout a very important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which result in instability or regulation errors. Power components such as the MAX1684/MAX1685 inductor, input filter capacitor, and output filter capacitor should be placed as close together as possible, and their traces kept short, direct, and wide, Connect their ground nodes in a star-ground configuration. Keep the extra copper on the board and integrate into ground as a pseudo-ground plane. When using external feedback, the feedback network should be close to FB, within 0.2 inch (5mm), and the output voltage feedback should be tapped as close to the output capacitor as possible. Keep noisy traces, such as those from LX, away from the voltage feedback network. Separate the noisy traces by grounded copper. Place the small bypass capacitors within 0.2 inch (5mm) of their respective inputs. The MAX1684 evaluation kit manual illustrates an example PC board layout, routing, and pseudo-ground plane. Connect AIN to IN with a short (0.2 inch) metal trace or a 1Ω resistor and bypass AIN to PGND with a 0.1µF capacitor. This acts as a lowpass filter to reduce noise at AIN. Pin Configuration TOP VIEW CVH 1 16 PGND AIN 2 15 SHDN 14 LX IN 3 CVL 4 AGND 5 MAX1684 MAX1685 REF 6 13 LX 12 BOOT 11 STBY FB 7 10 ILIM/SS CC 8 9 SYNC/PWM QSOP Chip Information TRANSISTOR COUNT: 2061 ______________________________________________________________________________________ 13 MAX1684/MAX1685 Inverting Output Interchanging the ground and VOUT connections yields a negative voltage supply (Figure 4). The component selections are the same as for a positive voltage converter. The absolute maximum ratings limit the output voltage range to -1.25V to +5.5V and the maximum input voltage range to 14V - VOUT. Low-Noise, 14V Input, 1A, PWM Step-Down Converters QSOP.EPS MAX1684/MAX1685 Package Information 14 ______________________________________________________________________________________ Low-Noise, 14V Input, 1A, PWM Step-Down Converters ______________________________________________________________________________________ MAX1684/MAX1685 NOTES 15 MAX1684/MAX1685 Low-Noise, 14V Input, 1A, PWM Step-Down Converters NOTES 16 ______________________________________________________________________________________